The invention relates to an apparatus for the contactless coupling of a suspended part to a force measuring device, in which the suspended part is located within a housing and is provided with a magnet and the device for measuring a force is provided with a controllable electromagnet, the latter being controlled in accordance with a measuring device for determining the reciprocal spacing of both magnets, i.e. with the aid of a control loop for maintaining this spacing.
Said apparatus is always used if a force measurement cannot or is not to take place in the housing, because e.g. the force measuring device cannot or should not be exposed to the conditions prevailing in the measurement area. This can be the case with corrosive media, or if incompatibility is caused by a corresponding pressure in the measurement area.
Such an apparatus for the contactless coupling of a suspended part to a force measuring device is e.g. described in DE-AS No. 11 64 114. The upper part of a load pan suspended on a balance acts through a housing on the lower part of the load pan constructed as a suspended part, the measurement of the position change of the suspended part relative to the upper part of the load pan taking place through the housing wall at the same point at which force coupling takes place with the aid of magnetic forces. At this point, a measured signal and a power signal are consequently superimposed and it is virtually impossible to exclude their reciprocal influencing. It is correspondingly difficult to work with this apparatus.
The same publication also proposes measuring the position change of the suspended part with respect to the force measuring device by using electro optical procedures, the reference system being fitted to the electromagnet, i.e. to the force measuring device. Although this modification virtually excludes disturbances, particularly of the measurement signal by a power signal, the housing must be made light-permeable at the corresponding point or points. It is common to both constructions, that the housing is subject to severe restrictions regarding the material selection at the coupling point or the passage point for the electro optical measuring device. In one case it is not even possible to use a diamagnetic or paramagnetic, electrically conductive material and in the other a distortion-free light permeability is required. Thus, measurements cannot take place under high pressure. In addition, only simple systems can be used for measuring the position change and they do not permit a differential measurement with oppositely superimposed action. The sinsitivity is correspondingly low and it is further reduced by the fact that spacing measurement must take place over a relatively great distance through the housing wall.
The problem of the invention is therefore to so modify an apparatus of the aforementioned type, that with respect to the measuring device for determining the position change of the suspended part there are no longer any restrictions regarding the design of the housing or such devices, so that the range of investigations and tests within the housing is drastically extended by the choice of the housing materials and the quality of the suspension regulation can be considerably improved by the use of more accurate position measuring devices.